Abstract:

In this work possible process improvements for biomass-fuelled small-scale combined heat and power (CHP) plants are evaluated and a new mixed integer nonlinear programming (MINLP) model for process synthesis optimisation of these processes is presented. Small-scale (1-20 MWe) CHP plants are of interest, as in Finland the potential to increase the CHP production is in converting small heating units to CHP production. However, the profitability of these small-scale CHP investments should be higher than today. The small-scale CHP plants are usually operated according to the heat demand of a district heating network or an industrial process. Thus, the possibility to increase the profitability of these plants is in improving their power-to-heat ratios (α) or electrical efficiencies (ηe).

The possibilities to increase the power production of small-scale CHP plants are studied here with simulations and optimisation. Especially, a superstructure of the possible process improvements and a MINLP model including the special properties of small-scale CHP plants is developed for the process synthesis optimisation. Unlike previous models of CHP processes, the model includes the modelling of pressures and steam and water property functions that depend both on temperatures and pressures. Also, a new model for a back-pressure steam turbine is developed. This model takes into account the nonlinear efficiency changes in the regulation stage of the turbine, the changes caused by the exhaust losses at the end of the turbine, and the dependence of pressure on the steam mass flow through the turbine at part loads. The part load operation of the small-scale CHP plants is incorporated into the model with multiperiods.

With the developed simulation and optimisation models the profitability of the process changes in small-scale CHP processes based on existing plants are evaluated. With the addition of a steam reheater, a feed water preheater, and a two-stage district heat exchanger the simulation and optimisation models found profitable processes where α is increased from 0.23-0.50 to 0.45-0.50, depending on the size of the plant. Similarly, ηe is increased in a profitable way from 0.17-0.30 to 0.28-0.30. If also natural gas is used as fuel and a gas engine integrated to the process, the efficiencies are further improved. In general, the process alternatives and model formulations presented here can be useful in the design and planning of new efficient small-scale CHP processes. Some of the model formulations can be utilised also in the modelling of other energy related processes with similar challenges as in the modelling of small-scale CHP plants.Tässä työssä arvioidaan mahdollisia sähköntuotantoa parantavia prosessimuutoksia biomassaa polttaville pienille yhdistetyn sähkön- ja lämmöntuotannon (CHP) laitoksille ja esitetään uusi jatkuvia ja kokonaislukumuuttujia sisältävä epälineaarinen (MINLP) malli näiden laitosten optimointiin. Suomessa potentiaalia CHP-tuotannon lisäämiseen on lähinnä pienen kokoluokan lämpölaitoksilla ja näiden muuntamisella yhdistettyyn sähkön- ja lämmöntuotantoon, joten pienet (1-20 MWe) CHP-prosessit ovat tällä hetkellä erityisen kiinnostavia. Jotta tarvittavat investoinnit saataisiin nykyistä kannattavammiksi, tulisi näiden pienten CHP-laitosten tehokkuuden parantua. Kaukolämpöverkon tai teollisuusprosessin lämmöntarve määrää usein pienten CHP-laitosten ajotavan. Mahdollisuus pienten CHP-laitosten kannattavuuden parantamiseen on nostaa niiden sähköntuotannon hyötysuhdetta (ηe) ja/tai rakennusastetta (α) eli tuotetun sähkömäärän suhdetta tuotettuun lämpömäärään.